This invention relates generally to the field of vehicular power steering systems and more particularly to a power steering system having a power fluid pump, a hydraulic cylinder for moving the steered wheels of the vehicle and a fluid controller between the pump and the hydraulic cylinder for controlling the flow of pressurized fluid therebetween.
Many vehicles presently being manufactured are equipped with power steering systems. Generally such systems include a power fluid pump which is driven by the engine, a hydraulic cylinder connected to mechanical steering linkage for moving the dirigible wheels and a fluid controller for controlling the flow of fluid between the pump and the hydraulic cylinder. The fluid controller is generally connected to or operated in conjunction with a movement of the steering wheel.
Vehicles which employ power steering systems include both on-the-road and off-the-road vehicles. "On-the-road" vehicles are those which are intended to be driven primarily upon streets, highways and the like and include passenger cars, trucks and buses. "Off-the-road" vehicles are those intended primarily for use other than on the public streets and highways and include farm equipment such as combines and construction equipment such as scrapers.
Some "on-the-road" vehicles and a great number of "off-the-road" vehicles are equipped with hydraulically operated mechanisms or implements such as loading devices, leveling or scraper blades, etc. Much farm equipment which is pulled by or connected to a tractor includes hydraulically operated implements.
In many instances it would be desirable to utilize the pressurized-fluid supply capabilities of the power fluid pump of the power steering system in connection with the operation of the hydraulically operated components or implements. If such arrangement were feasible only a single pump would be required for both the steering system as well as the hydraulic system servicing the implements thereby reducing not only the cost of manufacture but also the space requirements attributable to the pumps and pump drive mechanisms.
This has not generally been done, however, for the following reason. In many situations during operation of a vehicle having a power steering system as well as a hydraulically actuated implement, a steering maneuver must be undertaken at the same time the implement is being actuated. Thus pressurized fluid is required simultaneously by the hydraulic cylinder in the power steering system and the hydraulic cylinder which moves the hydraulically operated implement.
Depending upon the size of the pump, the requirements of the respective hydraulic cylinders and the magnitude of the steering maneuver and the movement of the implement, it may occur that the power steering system is supplied with less pressurized fluid than it would normally receive during a steering maneuver of similar magnitude. In such circumstances the driver of the vehicle will momentarily experience a loss of steering control, at least to some degree. This is due to the failure of the power steering system to respond in the manner expected by the driver, which is in turn caused by fluctuations in the pressure and quantity of fluid available at the steering cylinder.
Of course, in some situations a minor or perhaps even a major loss in steering control is of no particular moment. For example, in many farming operations a high degree of steering control by the driver of a tractor is not necessary and can be tolerated periodically when a hydraulically operated implement is actuated during a steering maneuver. On the other hand, there are situations in which any loss of steering control could represent a hazard.
Rather than depend upon the judgment of the operator in recognizing and reacting to those situations in which simultaneous actuation of the power steering system and power implement would be hazardous, most manufacturers of vehicles provide one power fluid pump for the power steering system and another pump for operating the hydraulic implements. While this ensures optimum steering control it also requires two pumps and two pump drive mechanisms and may also require additional piping in the hydraulic circuitry.
The present invention is addressed to the problem of utilizing only a single pump to service not only the power steering system but auxiliary hydraulic systems while avoiding any loss of steering control in the event that a steering maneuver is undertaken simultaneously with the actuation of an auxiliary hydraulic implement.